1. Field of the Invention
The present invention relates to a method of holding a monocrystal which allows a heavy monocrystal to be pulled irrespective of the strength of a seed crystal or a neck by mechanically holding a part of the crystal during the course of the growth of the same by a so-called Czochralski (CZ) method. The present invention also relates to a method of growing a monocrystal in which the above-described method of holding a monocrystal is utilized.
2. Description of the Related Art
The Czochralski method has been known as a method of manufacturing a semiconductor material such as silicon. As shown in FIG. 3A, according to this method, a seed crystal 52 held by a seed holder 51 is brought into contact with the surface of material melt 54 contained in a crucible 53. The seed crystal 52 is then pulled while being rotated. At this time, the pulling speed is controlled such that a neck 55 is formed below the seed crystal 52. Subsequently, there is formed a body 56 which is a monocrystal having a large diameter.
The formation of the neck 55 permits elimination of dislocations from the body 56 of the monocrystal formed after the neck 55. In recent years the weight of the crystal has tended to increase due to an increase in the diameter of a monocrystal and an improvement of manufacturing efficiency, and consequently the strength of the seed crystal 52 and the neck 55 have tended to become insufficient. If the crystal that is being grown falls as a result of fracture of the neck during the course of the pulling of the crystal, a serious accident may occur. In order to prevent such an accident, as shown in FIG. 3B, there has been adopted a method and apparatus which enable mechanical holding of a part of the crystal during the course of growth of the crystal.
In this apparatus, a stepped engagement portion 57 consisting of an increased-diameter portion and a reduced-diameter portion is formed between the neck 55 and the body (a straight cylindrical portion of a crystal) 56, and the crystal is pulled while lifting jigs 58, 58 hold the stepped engagement portion 57. Examples of such a technique are described in, e.g., Japanese Patent Application Laid-open (kokal) Nos. 62-288191, 63-252991, 3-285893, and 3-295893. For instance, in the apparatus disclosed in JP-A No. 3-285893, a stepped engagement portion is formed while a seed crystal is pulled, and when the stepped engagement portion reaches a position corresponding to gripping levers disposed at a predetermined height, the gripping levers grip and pull the stepped engagement portion.
In practice, the timing at which the crystal being grown is mechanically held cannot be simply determined for several reasons, as will be described later, thereby posing a considerable problem.
More specifically, a part of a crystal that is being grown cannot be mechanically held before the crystal has been grown to a certain extent. However, in terms of prevention of the fall of the crystal due to fracture of the neck, it is desirable to hold the crystal as soon as possible, before the crystal grows to become heavy.
In order to achieve the above, the crystal may be held immediately after the portion of the crystal to be chucked is formed during the growth of the crystal. However, in this case, the crystal must be held right above the material melt, so that a holding device is directly exposed to the high-temperature material melt (a temperature not less than 1400.degree. C. for silicon), thereby resulting in faulty operations or alteration of the quality of the material of the holding device. Further, the material melt may be contaminated by impurities.
If the crystal that is being grown is subjected to mechanical stress while still remaining at elevated temperature, plastic deformations may arise in the crystal, which in turn would cause slip dislocation in the crystal being grown. If such slip dislocation is generated in the crystal that is being grown, the mechanical strength of the area of the crystal where the slip dislocation is generated decreases, thereby imposing a risk of fracture for that area when the crystal has grown to become heavy.
Further, if the crystal is grown to be equal to or greater than a certain weight, the seed crystal or the neck may not bear the weight and hence may fracture. For this reason, it is necessary to grow the crystal to a weight less than the critical weight of the crystal being grown or to mechanically hold the crystal before the weight of the crystal has reached the critical weight.
Particularly in recent years, the diameter of a monocrystal grown by the Czochralski method has become increasingly greater with an increase in the degree of integration of semiconductor devices. For silicon, there is demand for a monocrystal having a diameter of greater than 8 inches, particularly a monocrystal having a diameter of greater than 12 inches.
For the case of the growth of such a crystal having a large diameter, even if the crystal is grown to a slight extent, the crystal will be grown to become heavy. Therefore, the crystal being grown must be mechanically held as soon as possible. In contrast, during the growth of a crystal having a large diameter, a high-temperature region inevitably extends over a wide area of the crystal. Therefore, before the crystal is grown to a certain extent, the temperature of the portion of the crystal to be held does not decrease to a temperature at which plastic deformation does not arise.
The critical weight of the crystal that is being grown is complicatedly affected by various factors, such as the geometry of the seed crystal or the neck, particularly the diameter, the quality of the crystal, the growing temperature and the types of stress exerted on the crystal (tensile stress, torsional stress, or bending stress). For this reason, accurate determination of the critical weight by calculation is difficult.